Objective Probabilities

Almost everyone will agree that when our background statistical knowledge is extensive enough, and when the case with which we are concerned is a repeatable event, then objective probabilities are appropriate, and these are the probabilities that should enter into computation of expectation and into our decision theory. A great many people will also agree that there is another whole class of cases, in which we are concerned with unique events, in which we lack statistical knowledge, and for which we must turn to subjective probability or one of its surrogates. I purpose here to argue against this distinction. Of course it is easy enough to argue this way in a purely philosophical vein every event must be unique--it has its own spatio-temporal locus and every unique event must belong to some class of events about which, in principle, we could have statistical knowledge. But this is not my point. My point is that from a down-to-earth practical point of view, from the point of view that seeks to compute probabilities and expectations for making decisions, the distinction between repeatable events is not only untenable but seriously misleading.